If President Donald Trump's hardline stance on illegal immigration leads to large-scale deportations, it could negatively affect the U.S. economy, some economists say.

Here's why:

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1. Immigrants make up a substantial portion of the workforce

Undocumented workers account for roughly 18 percent of employment in agriculture, 13 percent in construction and 10 percent at restaurants, hotels and casinos, according to a study done last year by the National Bureau of Economic Research.

The United States can't afford to suddenly lose vast numbers of the undocumented workers who pick fruit and vegetables, build houses, bus tables, staff meat-packing plants and clean hotel rooms.

2. Immigrants contribute to a growing supply of workers

Economists note that immigrants, including undocumented workers, play a vital role in the U.S. economy, and not only because they fill many low-paying jobs that Americans won't or can't do. The United States, like Japan and Western Europe, is being hobbled economically by an aging and slower-growing workforce. Economic growth depends on a steadily growing supply of workers.

3. Baby boomers are retiring

But aging baby boomers are retiring. And an unusually large proportion of prime-age men have stopped looking for work. Nearly eight years after the end of the Great Recession, the unemployment rate has fallen to 4.8 percent, near what economists consider full employment.

4. Younger Americans are going to college

As older Americans retire, younger ones are increasingly choosing to enroll in college rather than start work right out of high school. A result is that it's become harder for employers to fill the entry-level jobs that are often taken by undocumented immigrants.

Still, critics have often argued that low-wage immigrants in the United States end up depressing pay for everyone else. And economists have long wrangled over that possibility.

5. Deportation isn't cheap

Mass deportations could impose other costs as well. Gitis estimates that the government would have to spend $400 billion to $600 billion to deport all undocumented workers and prevent future illegal immigration.